The goal of this application is to define the mechanistic link between obesity, hormone silencing of the tumor suppressor GUCY2C, and colorectal cancer risk; the reversibility of this link; and the utility of oral GUCY2C hormone replacement to mitigate that risk. GUCY2C is the intestinal receptor for the paracrine hormone guanylin in the colorectum, the most commonly lost gene product in sporadic colorectal cancer in mice and humans. GUCY2C regulates intestinal homeostasis, and its silencing by paracrine hormone loss produces epithelial dysfunction characterized by hyperproliferation, increased DNA damage, and reprogramming of cell metabolism, increasing intestinal tumorigenesis. Unexpectedly, preliminary studies revealed that guanylin expression in colon is eliminated by obesity in mice and humans. Guanylin expression appears to be reversibly modulated by ingested calories, rather than by the endocrine, adipokine or inflammatory milieu associated with obesity. These observations suggest a model of cancer risk in which ingested calories contributing to obesity recapitulate established mechanisms underlying sporadic colorectal cancer, by suppressing guanylin expression, silencing the GUCY2C tumor suppressor and disrupting epithelial homeostasis, increasing tumorigenesis. In the present application, the first aim will test the hypothesis that obesity induces epithelial dysfunction and tumorigenesis by suppressing guanylin expression and silencing GUCY2C. These studies will establish suppression of guanylin expression as a critical mechanistic link between diet, obesity, and cancer risk. The second aim will define the contribution of reversible calorie- dependent modulation of guanylin expression to epithelial dysfunction in obesity. These studies will expand the current paradigm of cancer risk beyond obesity and its associated endocrine milieu, to include the role of ingested calories as a reversible risk factor linking obesity and cancer. Finally, the third aim will explore the utility of oral GUCY2C ligand supplementation to prevent obesity-induced epithelial dysfunction and colorectal cancer. These studies will establish the utility of oral GUCY2C ligand replacement as a chemopreventive strategy to mitigate obesity-related cancer risk. Together, these studies will define one concrete mechanism linking obesity to cancer, serving as a bridge between identification of risk factors (obesity, diet, ingested calories) and the molecular biology of cancer development through silencing of the GUCY2C tumor suppressor. Understanding these mechanisms underlying cancer risk posed by obesity will provide new strategies for countering these risks, including calorie restriction and oral hormone replacement. The potential for immediate translation of these results to mitigate colorectal cancer risk in obese patients can be appreciated in the context of the imminent regulatory approval of oral GUCY2C ligands to treat constipation.